EAST PITTSBURGH, Pa. -- A lawyer for the family of a 17-year-old fatally shot by a Pennsylvania police officer seconds after he fled a traffic stop said the boy did not pose a threat to anyone.

Civil rights attorney S. Lee Merritt said in a statement late Wednesday that he doesn't see any apparent justification for the use of deadly force by an East Pittsburgh police officer that left Antwon Rose Jr. dead. Allegheny County police are conducting an independent investigation of the shooting in East Pittsburgh, a borough about 10 miles (16 kilometers) east of Pittsburgh.

The shooting, part of which was captured on video and posted to Facebook by a bystander, has sparked some social media outrage, including from a handful of celebrities such as rapper Nas calling for punishment of the officer. A protest at the East Pittsburgh police headquarters involving more than a hundred people, some of whom laid down in front of a police cruiser, lasted for several hours Wednesday night.

Investigators said the officer stopped the car Rose and two other people were riding in Tuesday because it matched the description of a car reported to be involved in a shooting about 15 minutes earlier in a nearby town. As the officer took the driver into custody, the short video shows Rose and the other passenger running from the car.

The officer quickly fired three shots, all of which struck Rose, who later died at a hospital from his injuries. The medical examiner has not said where the teen was struck.

"We must emphasize that rumors of (Antwon) being involved in a separate shooting are unsubstantiated," Merritt wrote in his statement, saying the use of deadly force seems unjustified. "We know that he was not armed at the time he was shot down, that he posed no immediate threat to anyone."

Allegheny County Police Commissioner Coleman McDonough said Wednesday that he is confident the car Rose was in was involved in the earlier shooting, partly because a window had been shot. He said officers found two guns in the car, and that the driver was released after questioning without being charged.

McDonough confirmed that Rose was not armed and that no shots were fired at the officers.

Investigators and city officials have declined to name the officer, who has been placed on administrative leave. East Pittsburgh Mayor Louis Payne says the officer had worked for the department for two weeks and was officially sworn in a few hours before the shooting. Payne said the officer has eight years of prior law enforcement experience.

Leaders of the Black Political Empowerment Project, a Pittsburgh-based group that advocates for participation by black citizens in the political process, asked Thursday that the Pennsylvania attorney general's office investigate this and other police-involved shootings, citing only two instances where the Allegheny County district attorney has filed homicide charges against police officers in fatal shootings in the last 20 years.

Rose's friends, family, and teachers said he was a promising student, who volunteered at a charity, was generous and had a "million-dollar smile."

His cousin Theresa Lynn Rose Monroe said, "It's just hurting me. It's senseless, he was no threat, and I just don't understand why? Why's it got to keep happening?"